The present invention relates to a thermal printer for performing multicolor printing and, more particularly, to a thermal printer in which printing is performed using multicolor heat-sensitive paper obtained by coating base paper with a plurality of color development layers.
This thermal printer has a thermal line head constituted by heating elements arrayed in a direction perpendicular to the feed direction of, e.g., multicolor heat-sensitive paper. The thermal printer drives these heating elements to print dots for one line.
Dual-color heat-sensitive paper, for example, of a discoloration type and an additive color type are conventionally known as the multicolor heat-sensitive paper. The discoloration type heat-sensitive paper has a structure in which three layers, i.e., a red color development layer, discoloration layer, and black color development layer are stacked on base paper. The additive color type heat-sensitive paper has a structure in which black and red color development layers are stacked on base paper. Each of these discoloration and additive color type heat-sensitive paper is coated with a coating layer to prevent a yellowish surface due to long-term storage or the like. The heating elements of the thermal head heat such heat-sensitive paper to form red or black dots constituting an image such as a character or graphic pattern. The red dot is obtained by heating the heat-sensitive paper with a red developing thermal energy for melting a red color development layer, while the black dot is obtained by heating the heat-sensitive paper with a black developing thermal energy for melting the black color development layer.
In the above thermal printer, an image including, e.g., red and black characters can be printed in a batch printing scheme of forming both the red and black characters or in a division printing scheme of forming one of the red and black characters first and then forming the other.
In either printing scheme, the thermal printer drives heating elements corresponding to each red character with a heat current corresponding to the red developing thermal energy. Similarly, the thermal printer drives heating elements corresponding to each black character with a heat current corresponding to the black developing thermal energy. The thermal energy applied to the heat-sensitive paper depends on a product of the heat current and the heat period. When the feed speed of the heat-sensitive paper is made constant to determine the heat period, the thermal line head is heated up to a higher temperature during the printing of black characters than that obtained during the printing of red characters. This results in that the quality of printing is degraded due to deterioration of the head while the service life of the head is undesirably shortened.